FAQ

                Frequently Asked Questions (FAQ) for PostgreSQL
                                       
   Last updated: Fri Mar 11 16:42:06 EST 2005
   
   Current maintainer: Bruce Momjian (pgman@candle.pha.pa.us)
   
   The most recent version of this document can be viewed at
   http://www.postgresql.org/files/documentation/faqs/FAQ.html.
   
   Platform-specific questions are answered at
   http://www.postgresql.org/docs/faq/.
     _________________________________________________________________
   
                             General Questions
                                      
   1.1) What is PostgreSQL? How is it pronounced?
   1.2) What is the copyright of PostgreSQL?
   1.3) What platforms does PostgreSQL support?
   1.4) Where can I get PostgreSQL?
   1.5) Where can I get support?
   1.6) How do I submit a bug report?
   1.7) What is the latest release?
   1.8) What documentation is available?
   1.9) How do I find out about known bugs or missing features?
   1.10) How can I learn SQL?
   1.11) How do I join the development team?
   1.12) How does PostgreSQL compare to other DBMSs?
   
                           User Client Questions
                                      
   2.1) What interfaces are available for PostgreSQL?
   2.2) What tools are available for using PostgreSQL with Web pages?
   2.3) Does PostgreSQL have a graphical user interface?
   
                          Administrative Questions
                                      
   3.1) How do I install PostgreSQL somewhere other than
   /usr/local/pgsql?
   3.2) How do I control connections from other hosts?
   3.3) How do I tune the database engine for better performance?
   3.4) What debugging features are available?
   3.5) Why do I get "Sorry, too many clients" when trying to connect?
   3.6) Why do I need to do a dump and restore to upgrade PostgreSQL
   releases?
   3.7) What computer hardware should I use?
   
                           Operational Questions
                                      
   4.1) How do I SELECT only the first few rows of a query? A random row?
   4.2) How do I find out what tables, indexes, databases, and users are
   defined? How do I see the queries used by psql to display them?
   4.3) How do you change a column's data type?
   4.4) What is the maximum size for a row, a table, and a database?
   4.5) How much database disk space is required to store data from a
   typical text file?
   4.6) Why are my queries slow? Why don't they use my indexes?
   4.7) How do I see how the query optimizer is evaluating my query?
   4.8) How do I perform regular expression searches and case-insensitive
   regular expression searches? How do I use an index for
   case-insensitive searches?
   4.9) In a query, how do I detect if a field is NULL? How can I sort on
   whether a field is NULL or not?
   4.10) What is the difference between the various character types?
   4.11.1) How do I create a serial/auto-incrementing field?
   4.11.2) How do I get the value of a SERIAL insert?
   4.11.3) Doesn't currval() lead to a race condition with other users?
   4.11.4) Why aren't my sequence numbers reused on transaction abort?
   Why are there gaps in the numbering of my sequence/SERIAL column?
   4.12) What is an OID? What is a TID?
   4.13) Why do I get the error "ERROR: Memory exhausted in
   AllocSetAlloc()"?
   4.14) How do I tell what PostgreSQL version I am running?
   4.15) Why does my large-object operations get "invalid large obj
   descriptor"?
   4.16) How do I create a column that will default to the current time?
   4.17) How do I perform an outer join?
   4.18) How do I perform queries using multiple databases?
   4.19) How do I return multiple rows or columns from a function?
   4.20) Why do I get "relation with OID ##### does not exist" errors
   when accessing temporary tables in PL/PgSQL functions?
   4.21) What encryption options are available?
   
                            Extending PostgreSQL
                                      
   5.1) I wrote a user-defined function. When I run it in psql, why does
   it dump core?
   5.2) How can I contribute some nifty new types and functions to
   PostgreSQL?
   5.3) How do I write a C function to return a tuple?
   5.4) I have changed a source file. Why does the recompile not see the
   change?
     _________________________________________________________________
   
                             General Questions
                                      
    1.1) What is PostgreSQL? How is it pronounced?
    
   PostgreSQL is pronounced Post-Gres-Q-L, also called just Postgres.
   
   PostgreSQL is an object-relational database system that has the
   features of traditional commercial database systems with enhancements
   to be found in next-generation DBMS systems. PostgreSQL is free and
   the complete source code is available.
   
   PostgreSQL development is performed by a team of mostly volunteer
   developers spread throughout the world and communicating via the
   Internet. It is a community project and is not controlled by any
   company. To get involved, see the developer's FAQ at
   http://www.postgresql.org/files/documentation/faqs/FAQ_DEV.html
   
    1.2) What is the copyright of PostgreSQL?
    
   PostgreSQL is distributed under the classic BSD license. It has no
   restrictions on how the source code can be used. We like it and have
   no intention of changing it.
   
   This is the BSD license we use:
   
   PostgreSQL Data Base Management System
   
   Portions Copyright (c) 1996-2005, PostgreSQL Global Development Group
   Portions Copyright (c) 1994-1996 Regents of the University of
   California
   
   Permission to use, copy, modify, and distribute this software and its
   documentation for any purpose, without fee, and without a written
   agreement is hereby granted, provided that the above copyright notice
   and this paragraph and the following two paragraphs appear in all
   copies.
   
   IN NO EVENT SHALL THE UNIVERSITY OF CALIFORNIA BE LIABLE TO ANY PARTY
   FOR DIRECT, INDIRECT, SPECIAL, INCIDENTAL, OR CONSEQUENTIAL DAMAGES,
   INCLUDING LOST PROFITS, ARISING OUT OF THE USE OF THIS SOFTWARE AND
   ITS DOCUMENTATION, EVEN IF THE UNIVERSITY OF CALIFORNIA HAS BEEN
   ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
   
   THE UNIVERSITY OF CALIFORNIA SPECIFICALLY DISCLAIMS ANY WARRANTIES,
   INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
   MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. THE SOFTWARE
   PROVIDED HEREUNDER IS ON AN "AS IS" BASIS, AND THE UNIVERSITY OF
   CALIFORNIA HAS NO OBLIGATIONS TO PROVIDE MAINTENANCE, SUPPORT,
   UPDATES, ENHANCEMENTS, OR MODIFICATIONS.
   
    1.3) What platforms does PostgreSQL support?
    
   In general, any modern Unix-compatible platform should be able to run
   PostgreSQL. The platforms that had received explicit testing at the
   time of release are listed in the installation instructions.
   
   Starting with version 8.0, PostgreSQL now runs natively on Microsoft
   Windows NT-based operating systems like Win2000, WinXP, and Win2003. A
   prepackaged installer is available at
   http://pgfoundry.org/projects/pginstaller. MSDOS-based versions of
   Windows (Win95, Win98, WinMe) can run PostgreSQL using Cygwin.
   
   There is also a Novell Netware 6 port at http://forge.novell.com, and
   an OS/2 (eComStation) version at
   http://hobbes.nmsu.edu/cgi-bin/h-search?sh=1&button=Search&key=postgre
   SQL&stype=all&sort=type&dir=%2F.
   
    1.4) Where can I get PostgreSQL?
    
   The primary anonymous ftp site for PostgreSQL is
   ftp://ftp.PostgreSQL.org/pub/. For mirror sites, see our main web
   site.
   
    1.5) Where can I get support?
    
   The PostgreSQL community provides assistance to many of its users via
   email. The main web site to subscribe to the email lists is
   http://www.postgresql.org/community/lists/. The general or bugs lists
   are a good place to start.
   
   The major IRC channel is #postgresql on Freenode (irc.freenode.net).
   To connect you can use the Unix program irc -c '#postgresql' "$USER"
   irc.freenode.net or use any of the other popular IRC clients. A
   Spanish one also exists on the same network, (#postgresql-es), and a
   French one, (#postgresqlfr). There is also a PostgreSQL channel on
   EFNet.
   
   A list of commercial support companies is available at
   http://techdocs.postgresql.org/companies.php.
   
    1.6) How do I submit a bug report?
    
   Visit the PostgreSQL bug form at
   http://www.postgresql.org/support/submitbug.
   
   Also check out our ftp site ftp://ftp.PostgreSQL.org/pub/ to see if
   there is a more recent PostgreSQL version.
   
    1.7) What is the latest release?
    
   The latest release of PostgreSQL is version 8.0.0.
   
   We plan to have major releases every ten to twelve months.
   
    1.8) What documentation is available?
    
   PostgreSQL includes extensive documentation, including a large manual,
   manual pages, and some test examples. See the /doc directory. You can
   also browse the manuals online at http://www.PostgreSQL.org/docs.
   
   There are two PostgreSQL books available online at
   http://www.postgresql.org/docs/books/awbook.html and
   http://www.commandprompt.com/ppbook/. There is a list of PostgreSQL
   books available for purchase at
   http://techdocs.PostgreSQL.org/techdocs/bookreviews.php. There is also
   a collection of PostgreSQL technical articles at
   http://techdocs.PostgreSQL.org/.
   
   The command line client program psql has some \d commands to show
   information about types, operators, functions, aggregates, etc. - use
   \? to display the available commands.
   
   Our web site contains even more documentation.
   
    1.9) How do I find out about known bugs or missing features?
    
   PostgreSQL supports an extended subset of SQL-92. See our TODO list
   for known bugs, missing features, and future plans.
   
    1.10) How can I learn SQL?
    
   The PostgreSQL book at
   http://www.postgresql.org/docs/books/awbook.html teaches SQL. There is
   another PostgreSQL book at http://www.commandprompt.com/ppbook. There
   is a nice tutorial at
   http://www.intermedia.net/support/sql/sqltut.shtm, at
   http://ourworld.compuserve.com/homepages/graeme_birchall/HTM_COOK.HTM,
   and at http://sqlcourse.com.
   
   Another one is "Teach Yourself SQL in 21 Days, Second Edition" at
   http://members.tripod.com/er4ebus/sql/index.htm
   
   Many of our users like The Practical SQL Handbook, Bowman, Judith S.,
   et al., Addison-Wesley. Others like The Complete Reference SQL, Groff
   et al., McGraw-Hill.
   
    1.11) How do I join the development team?
    
   First, download the latest source and read the PostgreSQL Developers
   FAQ and documentation on our web site, or in the distribution. Second,
   subscribe to the pgsql-hackers and pgsql-patches mailing lists. Third,
   submit high quality patches to pgsql-patches.
   
   There are about a dozen people who have commit privileges to the
   PostgreSQL CVS archive. They each have submitted so many high-quality
   patches that it was impossible for the existing committers to keep up,
   and we had confidence that patches they committed were of high
   quality.
   
    1.12) How does PostgreSQL compare to other DBMSs?
    
   There are several ways of measuring software: features, performance,
   reliability, support, and price.
   
   Features
          PostgreSQL has most features present in large commercial DBMSs,
          like transactions, subselects, triggers, views, foreign key
          referential integrity, and sophisticated locking. We have some
          features they do not have, like user-defined types,
          inheritance, rules, and multi-version concurrency control to
          reduce lock contention.
          
   Performance
          PostgreSQL's performance is comparable to other commercial and
          open source databases. It is faster for some things, slower for
          others. In comparison to MySQL or leaner database systems, we
          are faster for multiple users, complex queries, and a
          read/write query load. MySQL is faster for simple SELECT
          queries done by a few users. Of course, MySQL does not have
          most of the features mentioned in the Features section above.
          We are built for reliability and features, and we continue to
          improve performance in every release.
          
   Reliability
          We realize that a DBMS must be reliable, or it is worthless. We
          strive to release well-tested, stable code that has a minimum
          of bugs. Each release has at least one month of beta testing,
          and our release history shows that we can provide stable, solid
          releases that are ready for production use. We believe we
          compare favorably to other database software in this area.
          
   Support
          Our mailing lists provide contact with a large group of
          developers and users to help resolve any problems encountered.
          While we cannot guarantee a fix, commercial DBMSs do not always
          supply a fix either. Direct access to developers, the user
          community, manuals, and the source code often make PostgreSQL
          support superior to other DBMSs. There is commercial
          per-incident support available for those who need it. (See FAQ
          section 1.5.)
          
   Price
          We are free for all use, both commercial and non-commercial.
          You can add our code to your product with no limitations,
          except those outlined in our BSD-style license stated above.
     _________________________________________________________________
   
                           User Client Questions
                                      
    2.1) What interfaces are available for PostgreSQL?
    
   The PostgreSQL install includes only the C and embedded C interfaces.
   All other interfaces are independent projects that are downloaded
   separately; being separate allows them to have their own release
   schedule and development teams.
   
   Some programming languages like PHP include an interface to
   PostgreSQL. Interfaces for languages like Perl, TCL, Python, and many
   others are available at http://gborg.postgresql.org in the
   Drivers/Interfaces section and via Internet search.
   
    2.2) What tools are available for using PostgreSQL with Web pages?
    
   A nice introduction to Database-backed Web pages can be seen at:
   http://www.webreview.com
   
   For Web integration, PHP (http://www.php.net) is an excellent
   interface.
   
   For complex cases, many use the Perl and DBD::Pg with CGI.pm or
   mod_perl.
   
    2.3) Does PostgreSQL have a graphical user interface?
    
   Yes, there are several graphical interfaces to PostgreSQL available.
   These include pgAdmin III (http://www.pgadmin.org, PgAccess
   http://www.pgaccess.org), RHDB Admin (http://sources.redhat.com/rhdb/
   ), TORA ( http://www.globecom.net/tora/, partly commercial), and
   Rekall ( http://www.rekallrevealed.org/). There is also PhpPgAdmin (
   http://phppgadmin.sourceforge.net/ ), a web-based interface to
   PostgreSQL.
   
   See http://techdocs.postgresql.org/guides/GUITools for a more detailed
   list.
     _________________________________________________________________
   
                          Administrative Questions
                                      
    3.1) How do I install PostgreSQL somewhere other than /usr/local/pgsql?
    
   Specify the --prefix option when running configure.
   
    3.2) How do I control connections from other hosts?
    
   By default, PostgreSQL only allows connections from the local machine
   using Unix domain sockets or TCP/IP connections. Other machines will
   not be able to connect unless you modify listen_addresses in the
   postgresql.conf file, enable host-based authentication by modifying
   the $PGDATA/pg_hba.conf file, and restart the server.
   
    3.3) How do I tune the database engine for better performance?
    
   There are three major areas for potential performance improvement:
   
   Query Changes
          This involves modifying queries to obtain better performance:
          
          + Creation of indexes, including expression and partial indexes
          + Use of COPY instead of multiple INSERTs
          + Grouping of multiple statements into a single transaction to
            reduce commit overhead
          + Use of CLUSTER when retrieving many rows from an index
          + Use of LIMIT for returning a subset of a query's output
          + Use of Prepared queries
          + Use of ANALYZE to maintain accurate optimizer statistics
          + Regular use of VACUUM or pg_autovacuum
          + Dropping of indexes during large data changes
            
   Server Configuration
          A number of postgresql.conf settings affect performance. For
          more details, see Administration Guide/Server Run-time
          Environment/Run-time Configuration for a full listing, and for
          commentary see
          http://www.varlena.com/varlena/GeneralBits/Tidbits/annotated_co
          nf_e.html and
          http://www.varlena.com/varlena/GeneralBits/Tidbits/perf.html.
          
   Hardware Selection
          The effect of hardware on performance is detailed in
          http://candle.pha.pa.us/main/writings/pgsql/hw_performance/inde
          x.html and http://www.powerpostgresql.com/PerfList/.
          
    3.4) What debugging features are available?
    
   There are many log_* server configuration variables that enable
   printing of query and process statistics which can be very useful for
   debugging and performance measurements.
   
   The following detailed debug instructions are to be used to provide
   more detailed information for server developers debugging a problem.
   
   It is also possible to debug the server if it isn't operating
   properly. First, by running configure with the --enable-cassert
   option, many assert()s monitor the progress of the backend and halt
   the program when something unexpected occurs.
   
   The postmaster has a -d option that allows even more detailed
   information to be reported. The -d option takes a number that
   specifies the debug level. Be warned that high debug level values
   generate large log files.
   
   If postmaster is not running, you can actually run the postgres
   backend from the command line, and type your SQL statement directly.
   This is recommended only for debugging purposes. Note that a newline
   terminates the query, not a semicolon. If you have compiled with
   debugging symbols, you can use a debugger to see what is happening.
   Because the backend was not started from postmaster, it is not running
   in an identical environment and locking/backend interaction problems
   may not be duplicated.
   
   If postmaster is running, start psql in one window, then find the PID
   of the postgres process used by psql using SELECT pg_backend_pid().
   Use a debugger to attach to the postgres PID. You can set breakpoints
   in the debugger and issue queries from psql. If you are debugging
   postgres startup, you can set PGOPTIONS="-W n", then start psql. This
   will cause startup to delay for n seconds so you can attach to the
   process with the debugger, set any breakpoints, and continue through
   the startup sequence.
   
   You can also compile with profiling to see what functions are taking
   execution time. The backend profile files will be deposited in the
   pgsql/data/base/dbname directory. The client profile file will be put
   in the client's current directory. Linux requires a compile with
   -DLINUX_PROFILE for proper profiling.
   
    3.5) Why do I get "Sorry, too many clients" when trying to connect?
    
   You have reached the default limit is 100 database sessions. You need
   to increase the postmaster's limit on how many concurrent backend
   processes it can start by changing the max_connections value in
   postgresql.conf and restarting the postmaster.
   
    3.6) Why do I need to do a dump and restore to upgrade between major
    PostgreSQL releases?
    
   The PostgreSQL team makes only small changes between minor releases,
   so upgrading from 7.4 to 7.4.1 does not require a dump and restore.
   However, major releases (e.g. from 7.3 to 7.4) often change the
   internal format of system tables and data files. These changes are
   often complex, so we don't maintain backward compatibility for data
   files. A dump outputs data in a generic format that can then be loaded
   in using the new internal format.
   
    3.7) What computer hardware should I use?
    
   Because PC hardware is mostly compatible, people tend to believe that
   all PC hardware is of equal quality. It is not. ECC RAM, SCSI, and
   quality motherboards are more reliable and have better performance
   than less expensive hardware. PostgreSQL will run on almost any
   hardware, but if reliability and performance are important it is wise
   to research your hardware options thoroughly. Our email lists can be
   used to discuss hardware options and tradeoffs.
     _________________________________________________________________
   
                           Operational Questions
                                      
    4.1) How do I SELECT only the first few rows of a query? A random row?
    
   To retrieve only a few rows, if you know at the number of rows needed
   at the time of the SELECT use LIMIT . If an index matches the ORDER BY
   it is possible the entire query does not have to be executed. If you
   don't know the number of rows at SELECT time, use a cursor and FETCH.
   
   To SELECT a random row, use:
    SELECT col
    FROM tab
    ORDER BY random()
    LIMIT 1;

    4.2) How do I find out what tables, indexes, databases, and users are
    defined? How do I see the queries used by psql to display them?
    
   Use the \dt command to see tables in psql. For a complete list of
   commands inside psql you can use \?. Alternatively you can read the
   source code for psql in file pgsql/src/bin/psql/describe.c, it
   contains SQL commands that generate the output for psql's backslash
   commands. You can also start psql with the -E option so it will print
   out the queries it uses to execute the commands you give. PostgreSQL
   also provides an SQL compliant INFORMATION SCHEMA interface you can
   query to get information about the database.
   
   There are also system tables beginning with pg_ that describe these
   too.
   
   Use psql -l will list all databases.
   
   Also try the file pgsql/src/tutorial/syscat.source. It illustrates
   many of the SELECTs needed to get information from the database system
   tables.
   
    4.3) How do you change a column's data type?
    
   Changing the data type of a column can be done easily in 8.0 and later
   with ALTER TABLE ALTER COLUMN TYPE.
   
   In earlier releases, do this:
    BEGIN;
    ALTER TABLE tab ADD COLUMN new_col new_data_type;
    UPDATE tab SET new_col = CAST(old_col AS new_data_type);
    ALTER TABLE tab DROP COLUMN old_col;
    COMMIT;

   You might then want to do VACUUM FULL tab to reclaim the disk space
   used by the expired rows.
   
    4.4) What is the maximum size for a row, a table, and a database?
    
   These are the limits:
   
   Maximum size for a database? unlimited (32 TB databases exist)
   Maximum size for a table? 32 TB
   Maximum size for a row? 1.6TB
   Maximum size for a field? 1 GB
   Maximum number of rows in a table? unlimited
   Maximum number of columns in a table? 250-1600 depending on column
   types
   Maximum number of indexes on a table? unlimited
   
   Of course, these are not actually unlimited, but limited to available
   disk space and memory/swap space. Performance may suffer when these
   values get unusually large.
   
   The maximum table size of 32 TB does not require large file support
   from the operating system. Large tables are stored as multiple 1 GB
   files so file system size limits are not important.
   
   The maximum table size and maximum number of columns can be quadrupled
   by increasing the default block size to 32k.
   
    4.5) How much database disk space is required to store data from a typical
    text file?
    
   A PostgreSQL database may require up to five times the disk space to
   store data from a text file.
   
   As an example, consider a file of 100,000 lines with an integer and
   text description on each line. Suppose the text string avergages
   twenty bytes in length. The flat file would be 2.8 MB. The size of the
   PostgreSQL database file containing this data can be estimated as 6.4
   MB:
    32 bytes: each row header (approximate)
    24 bytes: one int field and one text field
   + 4 bytes: pointer on page to tuple
   ----------------------------------------
    60 bytes per row

   The data page size in PostgreSQL is 8192 bytes (8 KB), so:

   8192 bytes per page
   -------------------   =  136 rows per database page (rounded down)
     60 bytes per row

   100000 data rows
   --------------------  =  735 database pages (rounded up)
      128 rows per page

735 database pages * 8192 bytes per page  =  6,021,120 bytes (6 MB)

   Indexes do not require as much overhead, but do contain the data that
   is being indexed, so they can be large also.
   
   NULLs are stored as bitmaps, so they use very little space.
   
    4.6) Why are my queries slow? Why don't they use my indexes?
    
   Indexes are not automatically used by every query. Indexes are only
   used if the table is larger than a minimum size, and the query selects
   only a small percentage of the rows in the table. This is because the
   random disk access caused by an index scan can be slower than a
   straight read through the table, or sequential scan.
   
   To determine if an index should be used, PostgreSQL must have
   statistics about the table. These statistics are collected using
   VACUUM ANALYZE, or simply ANALYZE. Using statistics, the optimizer
   knows how many rows are in the table, and can better determine if
   indexes should be used. Statistics are also valuable in determining
   optimal join order and join methods. Statistics collection should be
   performed periodically as the contents of the table change.
   
   Indexes are normally not used for ORDER BY or to perform joins. A
   sequential scan followed by an explicit sort is usually faster than an
   index scan of a large table.
   However, LIMIT combined with ORDER BY often will use an index because
   only a small portion of the table is returned. In fact, though MAX()
   and MIN() don't use indexes, it is possible to retrieve such values
   using an index with ORDER BY and LIMIT:
    SELECT col
    FROM tab
    ORDER BY col [ DESC ]
    LIMIT 1;

   If you believe the optimizer is incorrect in choosing a sequential
   scan, use SET enable_seqscan TO 'off' and run tests to see if an index
   scan is indeed faster.
   
   When using wild-card operators such as LIKE or ~, indexes can only be
   used in certain circumstances:
     * The beginning of the search string must be anchored to the start
       of the string, i.e.
          + LIKE patterns must not start with %.
          + ~ (regular expression) patterns must start with ^.
     * The search string can not start with a character class, e.g.
       [a-e].
     * Case-insensitive searches such as ILIKE and ~* do not utilize
       indexes. Instead, use expression indexes, which are described in
       section 4.8.
     * The default C locale must be used during initdb because it is not
       possible to know the next-greatest character in a non-C locale.
       You can create a special text_pattern_ops index for such cases
       that work only for LIKE indexing.
       
   In pre-8.0 releases, indexes often can not be used unless the data
   types exactly match the index's column types. This was particularly
   true of int2, int8, and numeric column indexes.
   
    4.7) How do I see how the query optimizer is evaluating my query?
    
   See the EXPLAIN manual page.
   
    4.8) How do I perform regular expression searches and case-insensitive
    regular expression searches? How do I use an index for case-insensitive
    searches?
    
   The ~ operator does regular expression matching, and ~* does
   case-insensitive regular expression matching. The case-insensitive
   variant of LIKE is called ILIKE.
   
   Case-insensitive equality comparisons are normally expressed as:
    SELECT *
    FROM tab
    WHERE lower(col) = 'abc';

   This will not use an standard index. However, if you create a
   expresssion index, it will be used:
    CREATE INDEX tabindex ON tab (lower(col));

    4.9) In a query, how do I detect if a field is NULL? How can I sort on
    whether a field is NULL or not?
    
   You test the column with IS NULL and IS NOT NULL.
   SELECT *
   FROM tab
   WHERE col IS NULL;

   To sort by the NULL status, use the IS NULL and IS NOT NULL modifiers
   in your WHERE clause. Things that are true will sort higher than
   things that are false, so the following will put NULL entries at the
   top of the resulting list:
   SELECT *
   FROM tab
   ORDER BY (col IS NOT NULL)

    4.10) What is the difference between the various character types?
    
        Type    Internal Name                    Notes
     VARCHAR(n) varchar       size specifies maximum length, no padding
     CHAR(n)    bpchar        blank padded to the specified fixed length
     TEXT       text          no specific upper limit on length
     BYTEA      bytea         variable-length byte array (null-byte safe)
     "char"     char          one character
   
   You will see the internal name when examining system catalogs and in
   some error messages.
   
   The first four types above are "varlena" types (i.e., the first four
   bytes on disk are the length, followed by the data). Thus the actual
   space used is slightly greater than the declared size. However, these
   data types are also subject to compression or being stored out-of-line
   by TOAST, so the space on disk might also be less than expected.
   VARCHAR(n) is best when storing variable-length strings and it limits
   how long a string can be. TEXT is for strings of unlimited length,
   with a maximum of one gigabyte.
   
   CHAR(n) is for storing strings that are all the same length. CHAR(n)
   pads with blanks to the specified length, while VARCHAR(n) only stores
   the characters supplied. BYTEA is for storing binary data,
   particularly values that include NULL bytes. All the types described
   here have similar performance characteristics.
   
    4.11.1) How do I create a serial/auto-incrementing field?
    
   PostgreSQL supports a SERIAL data type. It auto-creates a sequence.
   For example, this:
    CREATE TABLE person (
        id   SERIAL,
        name TEXT
    );

   is automatically translated into this:
    CREATE SEQUENCE person_id_seq;
    CREATE TABLE person (
        id   INT4 NOT NULL DEFAULT nextval('person_id_seq'),
        name TEXT
    );

   See the create_sequence manual page for more information about
   sequences.
   
    4.11.2) How do I get the value of a SERIAL insert?
    
   One approach is to retrieve the next SERIAL value from the sequence
   object with the nextval() function before inserting and then insert it
   explicitly. Using the example table in 4.11.1, an example in a
   pseudo-language would look like this:
    new_id = execute("SELECT nextval('person_id_seq')");
    execute("INSERT INTO person (id, name) VALUES (new_id, 'Blaise Pascal')");

   You would then also have the new value stored in new_id for use in
   other queries (e.g., as a foreign key to the person table). Note that
   the name of the automatically created SEQUENCE object will be named
   <table>_<serialcolumn>_seq, where table and serialcolumn are the names
   of your table and your SERIAL column, respectively.
   
   Alternatively, you could retrieve the assigned SERIAL value with the
   currval() function after it was inserted by default, e.g.,
    execute("INSERT INTO person (name) VALUES ('Blaise Pascal')");
    new_id = execute("SELECT currval('person_id_seq')");

    4.11.3) Doesn't currval() lead to a race condition with other users?
    
   No. currval() returns the current value assigned by your session, not
   by all sessions.
   
    4.11.4) Why aren't my sequence numbers reused on transaction abort? Why are
    there gaps in the numbering of my sequence/SERIAL column?
    
   To improve concurrency, sequence values are given out to running
   transactions as needed and are not locked until the transaction
   completes. This causes gaps in numbering from aborted transactions.
   
    4.12) What is an OID? What is a TID?
    
   Every row that is created in PostgreSQL gets a unique OID unless
   created WITHOUT OIDS. OIDs are autotomatically assigned unique 4-byte
   integers that are unique across the entire installation. However, they
   overflow at 4 billion, and then the OIDs start being duplicated.
   PostgreSQL uses OIDs to link its internal system tables together.
   
   To uniquely number columns in user tables, it is best to use SERIAL
   rather than OIDs because SERIAL sequences are unique only within a
   single table. and are therefore less likely to overflow. SERIAL8 is
   available for storing eight-byte sequence values.
   
   TIDs are used to identify specific physical rows with block and offset
   values. TIDs change after rows are modified or reloaded. They are used
   by index entries to point to physical rows.
   
    4.13) Why do I get the error "ERROR: Memory exhausted in AllocSetAlloc()"?
    
   You probably have run out of virtual memory on your system, or your
   kernel has a low limit for certain resources. Try this before starting
   postmaster:
    ulimit -d 262144
    limit datasize 256m

   Depending on your shell, only one of these may succeed, but it will
   set your process data segment limit much higher and perhaps allow the
   query to complete. This command applies to the current process, and
   all subprocesses created after the command is run. If you are having a
   problem with the SQL client because the backend is returning too much
   data, try it before starting the client.
   
    4.14) How do I tell what PostgreSQL version I am running?
    
   From psql, type SELECT version();
   
    4.15) Why does my large-object operations get "invalid large obj
    descriptor"?
    
   You need to put BEGIN WORK and COMMIT around any use of a large object
   handle, that is, surrounding lo_open ... lo_close.
   
   Currently PostgreSQL enforces the rule by closing large object handles
   at transaction commit. So the first attempt to do anything with the
   handle will draw invalid large obj descriptor. So code that used to
   work (at least most of the time) will now generate that error message
   if you fail to use a transaction.
   
   If you are using a client interface like ODBC you may need to set
   auto-commit off.
   
    4.16) How do I create a column that will default to the current time?
    
   Use CURRENT_TIMESTAMP:
    CREATE TABLE test (x int, modtime TIMESTAMP DEFAULT CURRENT_TIMESTAMP );

    4.17) How do I perform an outer join?
    
   PostgreSQL supports outer joins using the SQL standard syntax. Here
   are two examples:
    SELECT *
    FROM t1 LEFT OUTER JOIN t2 ON (t1.col = t2.col);

   or
    SELECT *
    FROM t1 LEFT OUTER JOIN t2 USING (col);

   These identical queries join t1.col to t2.col, and also return any
   unjoined rows in t1 (those with no match in t2). A RIGHT join would
   add unjoined rows of t2. A FULL join would return the matched rows
   plus all unjoined rows from t1 and t2. The word OUTER is optional and
   is assumed in LEFT, RIGHT, and FULL joins. Ordinary joins are called
   INNER joins.
   
    4.18) How do I perform queries using multiple databases?
    
   There is no way to query a database other than the current one.
   Because PostgreSQL loads database-specific system catalogs, it is
   uncertain how a cross-database query should even behave.
   
   contrib/dblink allows cross-database queries using function calls. Of
   course, a client can also make simultaneous connections to different
   databases and merge the results on the client side.
   
    4.19) How do I return multiple rows or columns from a function?
    
   It is easy using set-returning functions,
   http://techdocs.postgresql.org/guides/SetReturningFunctions
   .
   
    4.20) Why do I get "relation with OID ##### does not exist" errors when
    accessing temporary tables in PL/PgSQL functions?
    
   PL/PgSQL caches function scripts, and an unfortunate side effect is
   that if a PL/PgSQL function accesses a temporary table, and that table
   is later dropped and recreated, and the function called again, the
   function will fail because the cached function contents still point to
   the old temporary table. The solution is to use EXECUTE for temporary
   table access in PL/PgSQL. This will cause the query to be reparsed
   every time.
   
    4.21) What encryption options are available?
    
     * contrib/pgcrypto contains many encryption functions for use in SQL
       queries.
     * To encrypt transmission from the client to the server, the server
       must have the ssl option set to true in postgresql.conf, and an
       applicable host or hostssl record must exist in pg_hba.conf, and
       the client sslmode must not be disable. (Note that it is also
       possible to use a third-party encrypted transport, such as stunnel
       or ssh, rather than PostgreSQL's native SSL connections.)
     * Database user passwords are automatically encrypted when stored in
       the system tables.
     * The server can run using an encrypted file system.
     _________________________________________________________________
   
                            Extending PostgreSQL
                                      
    5.1) I wrote a user-defined function. When I run it in psql, why does it
    dump core?
    
   The problem could be a number of things. Try testing your user-defined
   function in a stand-alone test program first.
   
    5.2) How can I contribute some nifty new types and functions to PostgreSQL?
    
   Send your extensions to the pgsql-hackers mailing list, and they will
   eventually end up in the contrib/ subdirectory.
   
    5.3) How do I write a C function to return a tuple?
    
   In versions of PostgreSQL beginning with 7.3, table-returning
   functions are fully supported in C, PL/PgSQL, and SQL. See the
   Programmer's Guide for more information. An example of a
   table-returning function defined in C can be found in
   contrib/tablefunc.
   
    5.4) I have changed a source file. Why does the recompile not see the
    change?
    
   The Makefiles do not have the proper dependencies for include files.
   You have to do a make clean and then another make. If you are using
   GCC you can use the --enable-depend option of configure to have the
   compiler compute the dependencies automatically.